1. Field of the Invention
This invention relates to a fuel control system for an automobile engine, in particular an automotive vehicle engine having an air intake system into which fuel vapors are purged for feedback controlling an air-to-fuel ratio to maintain a stoichiometric air-fuel mixture.
2. Description of Related Art
Typically, fuel systems for engines, especially automotive vehicle engines, feedback control an air-to-fuel ratio by use of an oxygen (O.sub.2) sensor to maintain a stoichiometric air-fuel mixture.
On the other hand, it is typical for such an automotive vehicle engine to purge fuel vapors into an air intake system of the engine. The fuel vapors are usually stored in a canister and drawn into a surge tank forming part of the air intake system where they are mixed with fuel for burning whenever a prescribed purge execution condition is satisfied. A purge valve, provided between the canister and the air intake system, is controlled to open, permitting the fuel vapors to be drawn into the air intake system together with fresh air introduced into the canister. Such fuel vapor purge control is known from, for example, Japanese Unexamined Patent Publication No. 5-202815.
Purge gas introduced into the engine is a mixture of fuel vapors and air, and hence, when purging the fuel vapors together with performing the air-to-fuel ratio feedback control, it is necessary to subtract the amount of fuel vapors contained in the purge gas from the amount of fuel injected by an fuel injection valve. In order to determine the concentration of fuel in the purge gas, it is popular to learn the concentration of fuel in the purge gas based on an air-to-fuel ratio feedback control value in the air-to-fuel ratio control. In some types of fuel concentration learning, the learning gain in the purge gas concentration learning control is altered between when the air-to-fuel ratio feedback control value increases and when it decreases. Such fuel concentration learning is known from, for instance, Japanese Unexamined Patent Publication No. 6-323179.
When utilizing air-to-fuel ratio feedback control values to learn the concentration of fuel in a purge gas, the air-to-fuel ratio feedback control value serves as a parameter indicating a deviation of an air-to-fuel ratio from an ideally combustible or target air-to-fuel ratio for maintaining the stoichiometric air-fuel mixture. Specifically, the learned concentration of fuel in a purge gas is increased correspondingly to the learning gain if the air-to-fuel ratio feedback control value is on a side enriching the air-fuel mixture or decreased correspondingly to the learning gain if the air-to-fuel ratio feedback control value is on a side making the air-fuel mixture lean.
In the fuel control system for monitoring an actual air-to-fuel ratio and an air-to-fuel ratio feedback control value, there is a time-phase difference of, for example, approximately .pi./2 between them due to a time lag between fuel injection through a fuel injection valve and response of an oxygen (O.sub.2) sensor to a burnt gas. Consequently, inaccuracy of the concentration of fuel in a purge gas is encountered by direct use of an air-to-fuel ratio feedback control value as a parameter indicating a deviation of an air-to-fuel ratio from a target air-to-fuel ratio results, which is always undesirable for the precise air-to-fuel ratio control.